aggressively thinning latex paint?

Is it OK to aggressively thin latex paint with distilled water? My
aim in doing so is NOT to increase coverage, but rather to improve the
penetration into splits and cracks and rough areas. My intent would
be to allow this first thinned coat to soak in and dry thoroughly, then
apply two more coats of the same product, unthinned.
If the answer is "no", could you please give some technical explanation
why it is not a good idea.
I already tried this on some old treated pine deck boards I used to
repair portions of an exterior porch stairs. I washed and rinsed the
boards, then let them dry thoroughly. Then I painted them with an
exterior acrylic latex porch paint thinned 50/50 with distilled water.
This first coat had remarkable penetration. After it dried
thoroughly (a couple of days in 85 degree weather in the garage) I put
2 more coats of the same product, unthinned, allowing thorough drying
between coats. I used these boards to replace some worn exterior
porch stair treads, but they've only been in place for a couple of
weeks so far so I won't know the results for a couple of years.
In the past, I've used an oil primer, followed by 2 coats of latex
topcoat, but haven't had very good results. The latex bonded
tenaciously to the primer, but the primer blistered and peeled away
from the wood. This exterior application sees lots of sun, rain,
snow, and foot traffic.

oil base paint dosen't breathe so any moisture will lift the paint. latex
does so some moisture will come through without any issues. I have used
floetrol (sp) available at DD Lowes, and sherwin williams to thin it and
extend drying time (on Latex) which lets it penetrate more and although some
paints do not reccomend it I have not had a problem. Penetrol is for oil
based paints but not as easy to find

Thinning aggressively is a bad practice, you may
get the color to move into cracks but you end up
with mostly pigment and very little base to hold
the pigment to the wood. If you look at most
latex primers it says to not thin or to use a
maximum of 10 percent thinning fluid. BTW,
thinning with distilled water is overkill, any
drinkable water would be acceptable.
Latex paints don't really penetrate, they just lie
on the surface. So your idea of penetrating doing
something good is false. Paint won't fill in
cracks effectively. What you want to do is lay on
a coat that cover thoroughly to seal the wood by
coating it. If there are cracks you should be
sealing the cracks with a caulk before you paint.
Or, preferably, use sound board with no cracks.
A really good undercoat that dries on your hands
takes a lot of work to get it off, even if you
skin is oily. Your thinned paint will come off
your hands relatively easily.
What you need for a long lasting application is
not aggressive thinning, but aggressive coating.
The wood needs to be smoothed, cracks filled, and
the surface coated with a high quality porch paint.

Thanks for your lengthy response George. Some of your comments puzzle
me so I'd like to ask a few questions.
I don't understand your statement that I "end up with mostly pigment
and very little base to hold the pigment to the wood". If I thin
50/50, doesn't the ratio of base to pigment stay the same? In other
words, what happens to all the base? Isn't it still there, just like
the pigment is? Or is there some sort of chemical reaction where the
water destroys the base? I realize that by thinning, I don't get as
thick a coat as I would otherwise, but I figure that "something" is
better than "nothing", since the unthinned paint simply does not get
down into the fine cracks in the wood.
The cracks I am talking about are not large cracks that could be
caulked. I'm talking about many fine splits in the wood; of the order
of the thickness of a piece of paper. The unthinned paint simply
cannot get in there thoroughly, no matter how aggressively I brush it.
And, as it cures it leaves pinholes where the splits are; pinholes
where water could get in. If I thin the paint 50/50, it's still
fairly thick, but it is able to soak down into the splits instead of
just bridging over them. Then when I let it dry and paint over it with
unthinned paint, I get a continuous coating with no pinholes.
I tried scraping and washing a portion of the porch floor, then letting
it dry thoroughly and painting it with 50/50 thinned latex. You can
see the paint soak in to the fine splits, and when it dries it
absolutely doesn't rub off - it is very tenecious. I intend to cover
this first thinned coat with 2 additional coats using unthinned latex.
I'm not trying to go cheap on paint; I'm trying to get the wood coated.
Also, there are nooks and crannies in the porch where it's very
difficult to get the unthinned paint to go; a prime example is between
the deck boards. If I use unthinned paint, it wants to "bridge over"
adjacent boards instead of soaking down between them. Then, when you
walk on the boards, the slight relative motion between adjacent boards
causes the paint "bridges" to fail and expose bare wood. If I paint
these areas with thinned paint, it penetrates between the boards and
coats the hidden edges (where water drips down through). In this case,
when putting the unthinned overcoats in these areas, I would be
carefull to brush out any paint bridges.

The label says "do not thin" but it does not say *why*. Since I
intend to thin only the first coat, and then go over it twice with
unthinned paint, I would like to understand if (and why) thinning is
still objectionable.

I understand that this would be optimal. In another universe I'd love
to do that. But I don't have the time or money to rip up my porch
floor. I'm constrained to work with what I have, with the wood in
place. There is no rot, but in many areas the wood is weathered, and
the paint is blistering and peeling (actually, the latex topcoat is
adhering to the primer, but the oil primer is peeling from the wood).
It's because of my unsatisfactory experience with oil primer that I am
exploring other approaches.

In the painful process of learning, I've had the "pleasure" of using
several different paints, some of them awful. The one I am using now,
and with which I am fairly impressed, is "Best Look Premium 100%
Acrylic Latex Satin Porch & Floor Enamel" from the local hardware/paint
store. It says on the fine print on the label that it's made by
Sherwin Williams. It costs about 25 bucks a gallon. It is very thick
and creamy, easy to apply, and coats very well.

Sure the ratio is the same but the base needs to
be a certain concentration to provide a good a
reasonable coat, that is why manufacturers caution
against thinning. You don't need to get the paint
down in those thin cracks, you just need to be
sure that the paint seals over the cracks. I
suggest that you take a good board, and paint
small sections with 100%, 90%, 70%, 50% and 40%
paint thinning with water. Then scrub a dub with
water after 24 hours. That will show what the
paint you are using will do with thinning.

If that is true then do it. However,you would be
much better off filling the cracks with something
other than paint, maybe glue thinned about 10
percent (use Elmers carpenter glue (the yellow
stuff). Not only will you have the cracks mostly
filled they will be glued together.

You are trying to fix a structural problem with
paint. The board should not be moving in relation
to each other. If the boards are moving a tiny
bit you can seal with a flexible paint, but a
paint you walk on is not flexible to any extent.

Maybe you have the problem in hand. OTOH, the
peeling may be due to moisture absorption from
below, if the porch is over uncovered soil.

Good suggestion. I wasn't as scientific as you suggested with all the
different percentages, but I did run a "sanity" test before I committed
to using the thinned paint in my actual application.
I took a representative sample board and power-washed it to remove the
old blistering paint, then I let it dry and gave it a single coat of
aggressively thinned latex (I would guess 50/50 although I didn't
measure it). After letting it dry thoroughly I blasted it with the
power washer (1700 psi) and it didn't budge. So I figured, on that
basis, that using the thinned latex as a first coat, to be overcoated
with 2 coats of unthinned latex, was a reasonable thing to do. My
reason for posting here was to try to get some additional technical
reasons, if there are any, why the process I described is bad practice.
So far, to summarize what I am hearing, the gist of most of the
counter-arguments is that thinning the latex causes a thinner coat to
be applied, therefore resulting in a "weaker film". But no one so far
has suggested (at least not in clear language) that adding too much
water actually interferes with the paint's chemistry. Since I intend
to overcoat the first thinned coat with 2 coats of unthinned latex, it
seems to me that the "weakened film" argument is moot.

This is very interesting because this is exactly the sort of thing I
wanted to do initially, because it seems to make so much sense, but I
couldn't find _anybody_ to confirm the idea, despite talking to
contractors and paint-store people. Is this really a viable approach??
Would Elmers be the right stuff to use, or might there be something
even better? It has to be water-based for this project. Is Elmer's
really paintable?

My deck, which I built myself with 5/4" deck boards on 2x10 joists 16"
on center sitting on doubled-up 2x12 beams 6' on center supported by
posts every 5' along the beam length, you could drive a tank on it and
it wouldn't budge.
The porch is a different story. It is certainly not built as sturdily
as I would have done it myself, but it's not unlike many decks I have
walked on. The boards DO move slightly relative to one another when
you step on an area between joists. This absolutely kills the paint if
there are "paint bridges" joining the boards, as I discovered the hard
way. So when I re-paint it, I plan to avoid paint bridges between
adjacent boards.

Yes, it's over uncovered soil. It is going to be very interesting to
see what happens over time to the newly-refinished treads on the rear
stairs. I discussed this in a separate post in this thread. I used a
different process on each of the 11 treads and risers.

It sounds like you've invented primer. The unknown is how long it'll
last.
Why not just use a real primer? Oil-based primer should sink in better,
but even a latex primer will have a better chance of a long life. Both
can be topcoated with latex.

How is latex primer chemically different from latex topcoat paint?
Does it have a different binder system than latex paint?

Why not oil primer for this job? Here's why:
Many claim that oil-based primer penetrates better, but the caveat is
that the wood must be BONE DRY. The least bit of moisture appears to
greatly interfere with absorption and adherence. By contrast, latex
seems to be fairly forgiving: to apply latex, the wood can't be wet or
damp, but it doesn't have to be baked dry for a week. This is a
significant issue for this outdoor project, especially at this time of
year, when finding a whole week where there is no rain and the humidity
is low and there is no dew in the morning is next to impossible.
The area to be painted is a high-use area; it is a great inconvenience
to take it out of service. If I power-wash it, I have to keep traffic
off it until it dries or it will get dirty again. For oil primer, that
means a full week of dry weather after washing. If it rains, I have to
wait another week.
Once the wood is dry and the oil primer is applied, it takes several
days to dry properly so that it can be painted with latex. If it rains
during this period it seriously compromises the primer. All during
this period, the area must be off-limits to traffic.
Once the latex is applied, I need another 24 hours of dry weather or
the latex will be compromised.
On the other hand, if I use latex:
After power-washing, the wood is ready to accept a coat of latex within
24 to 48 hours. Within 4 hours I can apply a second coat. Within 24
hours it is rain-proof and ready for foot traffic. Done.
Besides the significant inconveniences of oil (as detailed above), my
experience will oil-based primer has been highly disappointing. See my
other posts in this same thread for more details, but here is the gist
of it. I tried using latex topcoat over oil primer previously and the
system failed within one year. The latex topcoat bonded to the oil
primer, but the oil primer started blistering and peeling away from the
wood in large chunks. I tried to follow all the rules. I do not know
if I somehow did something wrong, or if oil primer just isn't the right
solution for this particular application (exterior, wood, horizontal,
foot traffic, sun, rain, snow). Some boards were coated all six
sides, and some were coated on the top and ends only. All of them
failed prematurely.

I use a product called Floetrol for thinning Latex paints. It also adds a
lot of good properties like helping the paint flow. This is good for doors
when you want the paint to lay flat and not have brush marks. Available
most anywhere.
Steve

I think label instructions are good advice, and wouldn't recommend more
thinning than the label advises, especially for high wear and outdoors.
It might work, but why tempt fate for a tough condition? Primer should
not blister unless applied to damp or dirty wood or in hot, direct sun.
Start with clean, dry wood. Scrape and sand as needed. To fill cracks
(which invite disruption of paint film and intrusion of moisture), apply
paintable, flexible caulk after primer dries. Prime the caulk after it
dries. Paint. Cover all sides of the boards.
I don't buy cheap paint, and I hate paint prep work, but I am a fanatic
about the prep when I paint or paper because I don't want to have to do
it over. Why use poor quality paint, or ruin good quality paint? Paint
stores have product specifically for thinning both types of paint and
which don't weaken the film. The logic in not thinning too much is that
you dilute to the point that you are applying little more than colored
water. Of course, it would have "remarkable" penetration. Buy some new
wood or Trex.

"doesn't weaken the film" is the part I want to understand better. Why
doesn't this product "weaken the film" but water does? I'm not saying
you're wrong, I just want to understand how it works.

So... it doesn't matter if I thin the latex 50/50, as long as I apply
enough coats that the total amount of latex I use is the same as I
would have used if I didn't thin it?
For example, say I pour out two equal portions of unthinned latex into
separate containers. I paint one board with 2 coats using the first
portion (using it all up). I thin the second portion 50/50 with
distilled water, and paint the second board with that, applying coats
(and letting them dry) until the second portion is used up. Both
boards now have exactly the same amount of pigment and binder on them.
Will the paint on the second board fail because I thinned it? _Why_?
(It's the "why" part I am seeking to understand).

Look at the two. The product is every bit as thick as latex paint. It has
the consistency of latex paint. It has the same polymers and surfactants as
latex paint. It has added polymers and surfactants to help the paint flow
and "skin" properly. Pour a cup of it in your hand. Watch what it does.
Feel it.
Now do the same with a cup of water.
Here's yer sign.
Steve

I just looked at a container of Floetrol today. Until now, I'd never
heard of it.

Very cheap latex paint maybe. Floetrol is nowhere near as thick as the
latex paints I have been using.

How do you know this? The label on the container lists no ingredients.
The MSDS doesn't list any ingredients (it does say the boiling point
is 212F though). Where did you get your information?

Do you personally find that to be a satisfying technical answer to the
question?
If I paint one board with 8 ounces of high quality latex paint, using
as many coats as it takes to use up the paint, and I paint a second
board with 8 ounces of the same high quality latex paint plus 2 ounces
of distilled water, again using as many coats as it takes to use up the
10 ounces of thinned paint, are you saying the second board has a
"weaker" film? And if so, _why_?
OT, just heard on the news - Pluto has been stripped of its planet
status.

... and the boiling point of that 90% is, guess what? 212 degrees F.
Does that number sound familiar?

Apparently a tightly held secret? Steve B claims they're "polymers"
and surfactants. I wonder where he got that info. Seems weird that
they would put polymers in it. You'd think they'd use monomers, like
the original latex, so it would boost the binder.

Let me just say this. I have used it. I like it. It works for me. Your
mileage may vary. Use it or don't. It's as good a solution as any
proposed.
Surely as good as water. Maybe even a little better.
Steve

Floetrol is a latex medium with additional binders which are not in
water. It's good stuff. (I have a quart of it on my kitchen floor
waiting to be used with the latex enamel I'll use to repaint all the
interior house trim this month.)

Yes. Take an ounce of Elmer's glue. Thin it with water. Does it still
work as well? You've thinned out the chemical bonding. Ditto your
thinned paint. Most manufacturers ask you not to thin more than 10%
because it messes with the durability of their paint formula. You're
thinning 25-50% and it'll come back to bite you, ah gare-on-tee.
If you're low on engine oil in your car, it is recommended that you
don't add water to that, either. ;)

This just in: Everything We (You) Know Is Wrong. The sun isn't going
down, the horizon is moving UP!

Or not. I've read that link. There is no mention at all of polymers
or surfactants. All its says is that Floetrol contains "conditioners".
My shampoo contains "conditioners" too... maybe I could use that?
It's cheaper... :-)

It's also recommended in my car owner's manual that I do NOT add
third-party additives to my oil, either. Just like the label on my
latex which says "Do not thin"... not "Do not thin, except with
Floetrol, which, although made by our competitor, contains the exact
same chemistry as our proprietary acrylic binding system".
I'll probably pick up a gallon of Floetrol today and give it a try on
some of the boards I am refinishing and give it a fair try. It would
be nice to have more information about it though. Like what's in it.

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